The invention refers to a capacitive angular displacement transducer for the measurement of a rotor angle. It comprises three plates, two parallel stator plates and a rotor plate which is coaxially mounted between, and parallel to the first and second stator plate on a rotatable shaft. The first stator plate is divided into several conductive circle sectors of equal area which are electrically isolated from each other and fully cover a rotor angle of 2.pi. and are excited throughout with a plurality of electrical signals. The second stator plate which is parallel to the first consists of a concentric circle electrode for the generation of an electrical signal.
Furthermore, the invention refers to a measuring device for the measurement of a rotor angle which includes a capacitive angular displacement transducer with a generator for generating voltage signals that are connected to the circle sections of the first stator plate and an evaluation unit which is connected to the second stator plate.
Such a capacitive angular displacement transducer is described in U.S. Pat. No. 3,845,377. Its simple construction and simple signal processing unit which uses a charge amplifier, allows cost effective production. The signal evaluation is based on the phase difference between transmitted and received voltages on the sensor electrodes but without any error compensation mechanisms during the measurement. The major disadvantage of this sensor is that for most applications it is not precise enough.
DE-A-37 11 062 shows a further capacitive angular displacement transducer. This unit uses two identical stators each of which consists of excitation electrodes in the form of ring segments and a circular output electrode. The rotor, which is positioned between the two stators is also a combination of ring and circle segments. A configuration of this type on the one hand reduces the error due to rotor axis offset; on the other hand, however, it has the disadvantage that the close neighborhood of the excitation and the receiving electrodes causes interference between the transmitted and the received signals. Furthermore, a dielectric rotor may not be used resulting in an unchanged error susceptibility to axis tilt.
Error compensation by means of configuration in an angular displacement transducer especially on stator offset is suggested in U.S. Pat. No. 4,238,781 which also refers to angular displacement transducers with phase evaluation. In this solution, major disadvantages occur due to not taking into account stray fields into account and the necessity of two amplification units by which the precision of such a sensor is highly reduced. The method of including a multiplexer in the signal processing unit also results in a low precision angular displacement transducer since the input cables carry disturbing stray capacity.
Partial error compensation in capacitive angular displacement transducers is made possible by placing multiple electrodes on the circumference of the transducer as demonstrated in DD-A-13 213 081 especially for axis offset. Since this configuration only consists of two opposing plates, the output signal is sensitive to variation in distance between said plates. Therefore, it is only possible to evaluate the phases. In addition, this configuration leads to an unwanted coupling of the excitation signals to the output signal. A further disadvantage is that the full scale range of the transducer is less than 2.pi..
It is clear that in the field of linear and angular displacement transducers a higher resolution and accuracy can be obtained both through more specific configurations as well as a more complex signal processing. EP-A-258 725 for example demonstrates that the combination of fine and rough tracks enables high precision. The necessary electronics are however quite complex. Furthermore, such a capacitive angular displacement transducer has the following disadvantages: the individual segments of the fine track are so small that their behaviour is significantly influenced by stray fields. This leads to difficulties in achieving sinusoidal signals. This configuration requires a larger diameter as well. Due to the segmentation of the receiving electrodes, the sensitivity to axis tilt is low, but two amplifiers for each system are needed so that equal system behaviour is difficult to achieve. Since the active sector areas of both stators and the rotor are located on corresponding ring segments, the coupling varies with axis offset. On the fine track, error compensation is achieved by an at least double opposing configuration which is used primarily in optical encoders. Since four amplifiers are necessary, the placement of the electronics at a greater distance from the sensor is difficult. High enviromental temperatures necessitate the use of special and expensive amplifiers that can withstand high operating and environmental temperatures.
Among the encoders that work with electrostatic capacity, there are also those, as for example described in DE-A-35 38 455, which use a non-segmented and rotatable coupling electrode for signal transmission to the fixed output electrode. The coupling electrode is connected to the receiver electrode facing the transmitter electrode mounted on a fixed disc. This method does not substantially improve the insensitivity to axis tilt but the coupling electrode on the second side of the rotor eliminates the reciprocal influence of the pattern generator and the signal processing unit. Due to the relatively fine structure of the configuration, a reduction in transducer size is difficult while maintaining precision. Excitation i.e. providing the plurality of control voltages is very complex due to the large number of signals.